Differential mobilization of white adipose tissue fatty acids according to chain length, unsaturation, and positional isomerism.

The present study aims at determining whether, and how, the molecular structure of fatty acids influences their mobilization from fat cells. The in vitro mobilization of 52 fatty acids ranging in chain length from 12 to 24 carbon atoms, in unsaturation from 0 to 6 double bonds, and including 23 pairs of positional isomers was examined. Fat cells were isolated from adipose tissue of rats fed a fish-oil diet and treated with norepinephrine to stimulate lipolysis. Fatty acid composition of free fatty acids (FFA) released from these cells was compared to that of the triacylglycerols (TAG) from which they originated. The percentage weight of most fatty acids, and especially that of very long-chain fatty acids, was significantly different between FFA and TAG. The percentage of 20:5n-3 (eicosapentaenoic acid) and of 20:4n-6 (arachidonic acid) was 2.7 and 1.7 times higher in FFA than in TAG, respectively, whereas that of 20:1(n-11, 9 or 7), 22:1(n-11, 9 or 7) and 24:1n-9 was 1.7-2, 2.4, and 3.5 times lower, respectively. The relative mobilization (% in FFA/% in TAG) of the least readily mobilized fatty acid (24:1n-9) was 15-fold lower than that of the most readily mobilized (18:5n-3). For a given chain length, the relative mobilization increased exponentially with unsaturation, e.g., increasing from 0.45 to 2.7 in C 20 fatty acids when the number of double bonds increased from 0 to 5. Amongst the fatty acids with 18 to 22 carbon atoms, the shorter the chain was, the more steeply relative mobilization increased with unsaturation. On the other hand, for a given unsaturation the relative mobilization decreased with increasing chain length, e.g., decreasing from 1.15 to 0.3 in monoenes when the chain length increased from C 14 to C 24. A two-carbon-atom shortening of the chain length was on average equivalent to inserting one double bond for increasing the relative mobilization, i.e., by about 40%. The relative mobilization was also affected by the position of the double bond(s); increasing on average by 10% when there was a two-carbon-atom displacement towards the methyl end of the chain. These results demonstrate that under conditions of stimulated lipolysis individual fatty acids are more readily mobilized from fat cells when they are short and unsaturated, and when their double bonds are closer to the methyl end of the chain.(ABSTRACT TRUNCATED AT 400 WORDS)